Field of the Invention
The present invention relates to amplifier circuitry, and more particularly to a power amplifier with an automatic bias control.
Description of the Prior Art
A problem that exists in power amplifiers, and particularly in solid state audio power amplifiers, is that of excessive power dissipation in the output stages. As is well known to those skilled in the art, excessive power dissipation in the output stages tends to increase the temperature of the active output amplifier elements which in turn creates a regenerative effect tending to cause increased current flow and power dissipation. In extreme instances thermal runaway can result.
To reduce excessive power consumption and heat generation in the output stages, it has been conventional to select output biasing components of the proper magnitude to provide a low idle current and to ensure Class AB operation for low idle current and minimum heat generation. In practice, however, the proper selection of output biasing components to achieve these desirable ends is very difficult. Accordingly, the output biasing elements are often improperly selected with the result that either a high output idle current exists causing the generation of excessive heat in the output components, or Class B output conditions occur, with resulting distortion in the output signal due to crossover.
In high fidelity audio power amplifiers that are to be operated for long intervals neither excessive heat generation nor crossover distortions are permissible. Accordingly, a need exists for an improved biasing arrangement which eliminates both excessive current dissipation in the output stages and prevents distortion due to the occurrence of Class B output conditions.